The conjugated linoleic acids (hereinafter, collectively, "CLA"), a set of eight positional and geometric isomers of unconjugated cis-9, cis-12-octadecadienoic acid ("linoleic acid"), exhibit beneficial health effects when consumed in an animal's diet.
The cis-9, trans-11 and, to a lesser extent, the trans-10, cis-12 isomers, as well as other conjugated isomers, occur naturally in food. CLA was first isolated and identified from grilled ground beef extracts that exhibited anticarcinogenic activity (Ha et al., Carcinogenesis 8: 1881-1887, 1987). CLA is also found in some processed cheese products. Y. L. Ha, N. K. Grimm and M. W. Pariza, in J. Agric. Food Chem., Vol. 37, No. 1, pp. 75-81 (1987).
Synthetically prepared CLA inhibits chemically induced carcinogenesis in several animal model systems (Ha et al., 1996 supra; Ha et al., Cancer Res. 50: 1097-1101, 1990; Ip et al., Cancer Res. 51: 6118-6124, 1991; Ip et al., Cancer Res. 54: 1212-1215, 1994). Other biological activities seen in animal model systems include the reduction of adverse catabolic effects induced by immune stimulation (Miller et al., Biochem. Biophys. Res. Comm. 198(3): 1107-1112, 1994) and enhanced growth performance in rats (Chin et al., J. Nutr. 124: 2344-2349, 1994). CLA also reduces the development of atherosclerosis in rabbits (Lee et al., Atherosclerosis, 108: 19-25, 1994) and hamsters (Nicolosi et al., Circulation 88 Suppl: 2458, 1993) fed a high fat atherogenic diet containing cholesterol. It also reduces body fat content and increases lean body mass (Park et al., Abstract for 1995 Annual Meeting of the Institute of Food Technologists, 64-10: 183, 1995, and Park et al., Lipids 32:853-5 (1977)). These effects of CLA seem to be in part a consequence of effects on lipid metabolism.
By increasing the amount of dietary CLA, similar health effects could be realized in humans. One means by which dietary CLA could be increased is to increase the amount of CLA found in foods such as eggs. U.S. Pat. No. 5,504,114, incorporated herein by reference in its entirety, discloses that eggs can be markedly enriched for CLA by administering CLA to laying hens. However, U.S. Pat. No. 5,504,114 further discloses that when birds are fed a diet enriched in CLA their fertile eggs are unhatchable and evidence a dramatic alteration in the lipid profile. The CLA level in the yolk of eggs produced by laying hens fed a CLA-supplemented diet increased more than 20-fold; saturated fatty acids palmitate (16:0) and stearate (18:0) are increased, and monounsaturated fatty acids palmitoleate (16:1) and oleate (18:1) are decreased. The relative percentage of C16:0 and C18:0 increased about 37% and 87%, respectively. The relative percentage of C16:1 and C18:1 decreased about 48% and 41% respectively. The C16:0/C16:1 and C18:0/C18:1 ratios increased approximately 2.4 and 3.2 fold, respectively. In eggs having these changes in fatty acid composition, the egg yolk hardens when cooled to a temperature below room temperature (70.degree. F.), preferably to a temperature between 32.degree. F. and 70.degree. F., more preferably to a temperature between 32.degree. F. and 60.degree. F., and fertile eggs do not hatch when incubated.
In mammals, monounsaturated fatty acids are formed by directly oxidatively desaturating preformed long-chain (C.gtoreq.16) saturated fatty acids. Stearoyl-CoA desaturase ("SCD," also known as ".DELTA.9 desaturase") catalyzes the .DELTA.9 desaturation of palmitic (C16:0) and stearic acids (C18:0), forming palmitoleic (C16:1, n-7) and oleic (C18:1, n-9) acids. The desaturase enzyme is associated with the endoplasmic reticulum (microsomes) and can be isolated as microsomes from liver, mammary gland, brain, testes, and adipose tissues.
Many mechanisms may be involved in the process of regulating SCD activity, including dietary deprivation and alteration (Cook and Spence, J. Biol. Chem. 248: 1793-1796, 1973), hormones (Brenner, Biochem. Soc. Trans., 18: 773-775, 1990) and the composition of dietary fat (Christiansen et al., Biochim. Biophys. Acta 1082: 57-62, 1991; Ntambi, J. M., J. Biol. Chem. 267(15): 10925-10930, 1992). Diets rich in polyunsaturated fatty acids typically depress the activity of SCD (Christiansen et al., 1991 supra; de Antueno et al., Mol. Cell. Biochem. 118: 153-161, 1992; de Schrijver and Privett, J. Nutr., 112: 619-626, 1982; Garg et al., Biochem. Biophys. Acta 962: 330-336, 1988). The most convincing evidence indicates that decreases in SCD activity represent decreased enzyme synthesis arising from a reduction in the abundance of SCD1 mRNA (Ntambi, 1992 supra). A fat-free, high carbohydrate diet can increase SCD enzyme synthesis and the level of SCD1 mRNA in the liver (Landschulz et al.,Biochem. Biophys. Res. Comm., 200(2): 763-768, 1994; Ntambi, 1992 supra; Thiede and Strittmantter, J. Biol. Chem., 260: 14459-14463, 1985). Recent studies showed that arachidonic acid (C20:4) regulates monounsaturated fatty acid synthesis by inhibiting stearoyl-CoA desaturase gene expression in lymphocytes (Tebbey and Buttke, Biochim. Biophys. Acta., 1171: 27-34, 1992) and in mouse liver (Landschulz et al., 1994 supra).
It is commercially unacceptable to simply increase the amount of CLA in the diet of laying hens to enhance dietary CLA in eggs, because of the adverse effects that increased CLA levels have both on the ability of birds to reproduce and upon consumer acceptance of the eggs. What is needed in the art is a method for increasing the amount of CLA in eggs without causing undesired reproductive consequences for bird populations while maintaining an acceptable product for human use and consumption.